From the past, a moving-coil meter such as a circuit tester has been known as a device for displaying an electrical quantity by means of a hand which rotates within a sector-shaped region having a predetermined angle. Such an analog display system has gained much favor, since in comparison with a digital display system (numerical display), the analog display system has the merit that it is easy to read and allows the immediate and intuitive understanding of the magnitude of a displayed quantity. However, the moving-coil meter does not include a digital converting circuit for data but converts sustainedly the electrical quantity into rotary power of the hand. Thus, the meter consumes energy continuously, and hardly can it be incorporated in a miniatuarized instrument, for example, of a watch size, because of the restriction of the capacitance of the miniatuarized battery which is the incorporated power source.
On the other hand, as another example, there is a meter in which a physical quantity is converted into one or more driving signals in number in proporation to the physical quantity, then, the stepping motor is driven by the signals, next, rotary speed of the output shaft of the stepping motor is reduced by the gear train, and finally, the physical quantity is displayed by means of the hand attached to the shaft of one of the gears of the gear train. For driving the stepping motor, power may be instantaneously supplied only when reading is changed, and the physical quantity can be sampled at an arbitrary time interval and maintained within the digital circuit. Therefore, this meter is very advantageous from the viewpoint of power consumption. Also, some stepping motors have been extremely miniatuarized and their high power efficiency has been pursued. The most popular product in the meters of this type is a quartz oscillation type electronic watch, which displays the time as a physical quantity. Among the meters, there is a proposal for displaying a physical quantity other than time, using this hand display function (Japanese published unexamined utility model application No. 61-28019). However, in this prior art, since it is assumed that the hand is rotated without limit, a circular display region is required.
As further prior art, there is a proposal for displaying time information within an angular sector-shaped region by means of a hand which is driven by a stepping motor (Japanese published examined utility model application No. 63-17030). In this case, when time is full of the display range, the hand reaches the end of the display region, and as time further elapses, the hand has to be returned to the opposite end of the display region with a rapid motion. A driving signal for the stepping motor for doing this return action is generated by an electronic circuit which performs a logical action. Thus, once the hand is attached in the correct relation with respect to the electronically controlled hand driving mechanism, it is expected that the relation in phase between the electronic logical operation and the hand position cannot be displaced insofar the time display or the like which makes a regular repetition change is concerned. However, an effective method or system has not been provided for attaching the hand in a proper position (direction) in agreement with the logic state of the electronic circuit, especially when the movable range of the hand is mechanically restricted. Whatever physical quantities are to be measured, this synchronization is necessary and in order to mass-produce a device having a sector-shaped display function by use of a stepping motor, some effective synchronizing means are essential.
Furthermore, in the event that as in temperature and pressure, the physical quantity to be measured and displayed does not always fall within a predetermined display region and that the movable range of the hand is limited, a further problem will be posed. The reasons are as follows:
In the meter using a method of converting a physical quantity into the number of driving signals in proportion to the physical quantity and feeding the hand by means of the stepping motor, the absolute value of a physical quantity will not be given to the motor as stepping signals therefor (the method of returning the hand to the reference position every measuring sampling is possible but not preferable because it is inferior in traceability to the variations of measured values), and when physical quantities vary every moment, the difference between the last measured value and the new measured value, that is, incremental value, is given to the motor as the signals for the stepping motor so that the stepping motor can be fed in the forward or reverse direction by the difference. In this connection, one example of the methods for moving the stepping motor forward or backward at will is disclosed in U.S. Pat. No. 4,112,671.
Accordingly, suppose that the hand and its interlocking member hit a stop or obstacle and stop outside the measuring angle range, and then that an additional signal corresponding to the amount of the overtravel is applied. In that case, the stepping motor will be forced to stop as it is, even if the driving current flows. Furthermore, if at the next sampling, the physical quantity returns to the usual measurable range, a signal will be applied to the stepping motor for driving the hand additionally by the amount of the difference between the last overtraveled measured value and the present measured value. But as mentioned above, owing to the function of the stop, the hand is not located in the position where it should have been located and starts moving from the position where the stop has been hit. As a result, the next stopped position does not correspond to the correct measured value but points to a wrong position.
Also, in order to use a known structure generally used for analog quartz watches as a stepping motor (comprising a coil; a rotor having a permanent disc magnet magnetized so as to produce two poles across the diameter of the magnet; and a pair of yokes sandwiching the rotor at both sides and magnetically connected to the ends of the magnetic core of the coil, respectively), it is necessary to give bipolar driving pulses in which the polarity is reversed each step. Unless it is driven with the correct polarity, the hand will not move and a miscount will be caused. If the number of driving signals equal to the amount of the aforementioned overtravel for idling the stepping motor after the hand is blocked by the stop and rests, is an even number of steps, the hand will follow immediately by the first return pulse and performs return action, but if the number of driving signals equal to the amount of overtravel is an odd number, it is often that the hand will not follow by the first return pulse, thereby constituting a factor of miscounting which causes the misplacement of the hand.